fix crash in webenginepart on konqueror shutdown

Summary:
konqueror crashes on shutdown in webenginepart.

We fix this by not setting

QWebEngineProfile *prof = QWebEngineProfile::defaultProfile();

as parent of s_cookieJar.

BUG: 407454
FIXED-IN: 19.04.3

see also https://bugs.kde.org/show_bug.cgi?id=407454

Test Plan: start and shutdown konqueror without crash

Reviewers: Konqueror, dfaure

Reviewed By: Konqueror, dfaure

Subscribers: cfeck, anthonyfieroni

Differential Revision: https://phabricator.kde.org/D21562